The present invention relates to improved surface segregation of polysiloxane polyethers contained in a thermoplastic composition which is melt extruded to form fibers or nonwoven webs. Such improvement results from the use of a polypropylene-polyethylene block copolymer as the thermoplastic polymer component.
Surface-segregatable, melt-extrudable thermoplastic compositions are described in U.S. Pat. No. 4,923,914, SURFACE-SEGREGATABLE, MELT-EXTRUDABLE THERMOPLASTIC COMPOSITION, which issued in the names of Ronald S. Nohr and J. Gavin MacDonald. The patent describes a surface-segregatable, melt-extrudable thermoplastic composition which includes at least one thermoplastic polymer and at least one additive having at least two moieties, A and B, in which:
(A) the additive is compatible with the polymer at melt extrusion temperatures but is incompatible at temperatures below melt extrusion temperatures, but each of moiety A and moiety B, if present as separate compounds, would be incompatible with the polymer at melt extrusion temperatures and at temperatures below melt extrusion temperatures; PA1 (B) moiety B has at least one functional group which imparts to the additive at least one desired characteristic; PA1 (C) the molecular weight of the additive is in the range of from about 400 to about 15,000; and PA1 (D) the weight ratio of the polymer to the additive is in the range of from about 1 to about 1,000; PA1 with the proviso that the additive cannot be a compound having the general formula, ##STR1## in which each R independently is a monovalent organic group selected from the group consisting of alkyl groups; R.sup.1 is a monovalent organic group containing at least one ethyleneoxy group, vicinal epoxy group, or amino group; and a and b, which can be the same or different, each have a value of at least 1. In desired embodiments, the additive is a siloxane-containing compound, and one of the desired characteristics is wettability by water when the polymer is inherently hydrophobic. PA1 (b) R.sub.10 is hydrogen or a monovalent C.sub.1 -C.sub.3 alkyl group; PA1 (c) m represents an integer of from 1 to about 4; PA1 (d) n represents an integer of from 0 to about 3; PA1 (e) the sum of m and n is in the range of from 1 to about 4; PA1 (f) p represents an integer of from 0 to about 5; PA1 (g) x represents an integer of from 1 to about 10; PA1 (h) y represents an integer of from 0 to about 5; PA1 (i) the ratio of x to y is equal to or greater than 2; PA1 (j) the additive has a molecular weight of from about 350 to about 1,400; and PA1 (k) the additive is present in an amount of from about 0.1 to about 5 percent by weight, based on the amount of thermoplastic polyolefin. PA1 (A) a thermoplastic polypropylene-polyethylene block copolymer in which the polyethylene blocks constitute from about 1 to about 10 percent by weight of the copolymer and PA1 (B) an additive having the general formula, ##STR3## in which: (1) R.sub.1 -R.sub.7 are independently selected monovalent C.sub.1 -C.sub.3 alkyl groups; PA1 (2) R.sub.8 is hydrogen or a monovalent C.sub.1 -C.sub.3 alkyl group; PA1 (3) p represents an integer of from 0 to about 5; PA1 (4) x represents an integer of from 1 to about 15; PA1 (5) the additive has a molecular weight of from about 350 to about 1,400; and PA1 (6) the additive is present in an amount of from about 0.1 to about 5 percent by weight, based on the amount of thermoplastic block copolymer.
The compositions described in that patent are particularly useful for the formation of nonwoven webs by such melt-extrusion processes as meltblowing, coforming, and spunbonding. Upon being melt-extruded, such compositions result in a fiber (or film) having a differential, increasing concentration of the additive from the center to the surface thereof, such that the concentration of additive toward the surface of the fiber is greater than the average concentration of additive in the more central region of the fiber and imparts to the surface of the fiber at least one desired characteristic which otherwise would not be present.
When the additive was a siloxane-containing compound and the desired characteristic was water-wettability, the resulting nonwoven webs often became less wettable over time and frequently reverted to a nonwettable state. This loss of wettability, or aging, was accelerated when the polymer composition contained titanium dioxide. However, the absence of titanium dioxide did not prevent the aging which typically was complete within a matter of days.
A subclass of the additives encompassed by U.S. Pat. No. 4,923,914 subsequently was discovered which permitted the preparation of wettable nonwoven webs which remained wettable for at least about two years at ambient temperature. Such subclass is described and claimed in application Ser. No. 07/566,589, entitled SURFACE-SEGREGATABLE COMPOSITIONS AND NONWOVEN WEBS PREPARED THEREFROM and filed Aug. 13, 1990 in the names of Ronald S. Nohr and J. Gavin MacDonald.
Application Ser. No. 07/566,589 provides a surface-segregatable, melt-extrudable thermoplastic composition which includes at least one thermoplastic polyolefin and at least one additive having the general formula, ##STR2## in which: (a) R.sub.1 -R.sub.9 are independently selected monovalent C.sub.1 -C.sub.3 alkyl groups;
A particularly desirable subclass of the additives of application Ser. No. 07/566,589 includes trisiloxane polyethers, i.e., compounds having the above general formula in which m is 1 and n is 0.
It now has been discovered that the amounts of a particular group of trisiloxane polyethers in a thermoplastic composition can be significantly reduced without notably adversely affecting either the wettability of nonwoven webs prepared from such compositions or their long-term wettability characteristics. Moreover, the webs are immediately wettable without the need for a post-formation treatment of any kind.